The present invention relates to communications systems, and, more particularly, to a synchronization scheme for frequency division multiplexed communications.
Frequency division multiplexed (FDM) signals are characterized by a frequency band constituted by many signal channels occupying a respective sub-bandwidths. Signals carried by the individual channels can be encoded through a variety of schemes, e.g. frequency shift keying (FSK) or phase shift keying (PSK). Each signal comprises a stream of encoded bits characterized by a predetermined shared symbol period.
A system for processing FDM signals can include several components which need to be synchronized in some way with the FDM signal. Such synchronization can be fairly straightforward when the event to be synchronized can be initiated at the signal stream. In such a case, synchronization can be triggered using signal transitions within the FDM stream itself.
However, where the event to be synchronized occurs remotely from the FDM stream, synchronization can be more difficult. This is especially true where one or both of the FDM stream and the event to be synchronized encounter intermediate processing prior to convergence. In such a case, even when synchronization is achieved, it can be difficult to maintain due to timing changes such as can be caused by temperature variations or system aging.
By way of example, a frequency sweep must be synchronized with the FDM symbol periods in a surface acoustic wave (SAW) demodulator for converting received FDM signals to time division multiplexed (TDM) signals. The SAW demodulator can include three reflective array compressors (RACs) for time-dispersing signals as a function of frequency. An input RAC time-staggers one set of symbol segments of the channels in an FDM signal. The staggered segments are then frequency swept to produce a sequence of frequency sweeps, each sweep corresponding to a respective channel's symbol segment. An output RAC compresses these sweeps into a series of TDM pulses, each pulse reflecting the content of the corresponding channel's symbol segment.
For this FDM-to-TDM conversion to be performed reliably, the frequency sweep must be synchronized with the FDM signal in its staggered form at the exit of the input RAC. The frequency sweep itself can be formed by generating a pulse, then time dispersing the pulse as a function of frequency using a frequency sweep RAC, and then multiplying this sweep with the staggered FDM signal. The challenge is to generate the pulse at the right time so that the sweep and FDM signal converge with the correct timing.
The problem is that the delays introduced by the various RACs are variable and generally not precisely known at the outset. What is needed is a reliable scheme for synchronizing the sweep that can compensate timing changes due to temperature and other factors. Generally, what is needed is a reliable approach to synchronizing FDM processing events which themselves undergo processing or other delays prior to converging with the FDM stream.